Air build up in Forks

[James RookyardMember]

Actually, the feeler gauge was inserted and air escaped. The seal was actually sealing well. It was air tight. That would mean only the tiniest bit of crud would have let oil out. So yes air pressure build up would make them leak easier, but I don’t think it would make the seal blow. It was the often the done thing in the earlier years to “pump up” the forks through the bleeder valves. I can remember ADB forever telling people…. the valves are to let air out, NOT to put air in.
When the fork compresses the air space allows compression. You can’t compress a liquid so the air compresses in the fork. The seals handle this no worries. They get hot when you ride, the air pressure will increase as the air expands. So you let the air pressure out when you finnish riding. When the air cools again no doubt a slight vacuum occurs in the fork leg due to the air volume having been decreased. So as soon as you move the bike air is sucked in to the fork to equalise the air pressure(perhaps this is when crap starts to get drawn up under the seal?). When it gets hot you now have even more air in the leg than the last time you rode so it builds up even more pressure than the last time you rode it.
Perhaps the problem is the bleeder valves. You have no idea if the shock is building air pressure when you ride, because they don’t have a bleeder valve. They must do the same thing though.
feel free to shoot holes in my theory any suspension experts out there lol.

[Nick DoleMember]

Lot of theories here. My 5c.
I can only answer from what Ive seen and done.
I don’t like bleeders that screw in place of bleed screws. They can leak, both vacuum and pressure and they are susceptible to being snapped off from tie downs and flying rocks. the screw is less glamorous but it’s 100% reliable and free with the bike.
Regardless of the type the pressure needs to be equalised with the front wheel off the ground. The air in the fork is a secondary spring to your main spring (comes under Boyles law for those with the inclination to research it) If you equalise with the fork compressed you lessen the force of the air spring so the fork will become softer. there is a chance the seals could actually be sucked ‘in” but I’ve never seen it. Wives tail.
In the past we have used air pressure to stiffen the overall fork action. It’s not a great solution as the fork becomes very stiff initially. The fork will happily run with 40 psi in it, your wrists won’t like it much though….
The manufacturers deleted the use of Schrader valve fittings when they woke up and realised no one was using them to add pressure.
In 2013 the air fork is back in the KX and CRF450. NO SPRINGS just air. 32-36 psi. they get around the inital harshness by using very long/stiff top out springs to oppose the main springs. 2 guys ran the KX450 at the 4 day this year, both DNF’d because the forks would not hold air for a whole special test, even with a pump in the bumbag. Different type of seal to our current seal as the high pressure makes seal drag a big problem.
I do some hydraulics work myself (skid steer loader, was making new pins and bushes for the 4 in one yesterday) and one of my friends is a tool maker who specialises in making/repairing hydraulic rams (XR650 nut too, putting a 916 Ducati swingarm in it) sliding friction is no problem for hydraulics, gland seals are meant to be bloody tight. how would that feel in your forks?
As for the brake side leaking first, yes there is some twisting motion involved, brake dust does not help either.
Bottom line, with the low friction fork seals we all use the working life is short, manufacturers suggest 10 hours, most of us can get 40 if we do everything right. Slapping a new set of seals only, no wipers, not polising the fork tubes and not having a really good look and feel of the bushes is not a fork service, that’s another leaking seal waiting to happen.

[Gareth Morgan-ThomasMember]

And so endeth the lesson!!!!

[AlexMember]

Whammo!!!!
I also have a mechanical engineer that rebuilds hyd cylinders for a living, I’ll spare him the delight. This one is done and dusted.

[Dwayne OMember]

Yeah ,, Thanx for the insight Nick ,,,
Sounds reasonable enough to me

I have been equalising my forks on the stand for the past 18 months and thought it was a good thing :whistle:
Now I know :woohoo:

[James RookyardMember]

I assume the air build up is getting in past the seals? How come its not an issue on the rear shock as well?

[Nick DoleMember]

Air get in past the seals when dirt starts to accumulate in the seal. it’s the first sign you about to get a leaking seal. the amount of air that escapes from the bleeders increases, then the seal starts to weep or can “blow” if there is an air build up in the fork and the pressure pushes the oil out.
Forks are more prone to leaking because the seal is not self energised, there is a steel spring in the seal to keep the lip in contact with the shaft. A shock seal is self energised, the 150psi plus in the shock pushes the seal lip into the shaft. if I service a shock and left it overnight with no gas it will leave a puddle on the bench the next day, the seal needs the be energised to work, as do the o rings in the shock.
Secondly, the fork is in a much harsher environment. there is mud and dirt being sprayed onto the fork tubes constantly, flying rocks, crash damage. The dust wiper can only remove so much dirt, some of it makes it’s way into the seal.
In contrast, the shock shaft is well protected from debris and crash damage. if you remove the flap that protects the shock shaft from the rear wheel roost the shaft is peppered very quickly and the shock leaks shortly after.

[James RookyardMember]

Ah that makes sense.

[Matt BakerMember]

Moral to the story, “clean and protect your shaft” :laugh: :laugh: :laugh:

[Jeffrey SmithMember]

LC4skin wrote:

Moral to the story, “clean and protect your shaft” :laugh: :laugh: :laugh:

Always :laugh: :blush:

[James RookyardMember]

My 96 CR250 is sitting in the shed, guess which fork seal has started leaking lol.

[Nick DoleMember]

Good bike that 96, took them until 2003 to make a better one

[James RookyardMember]

Still nice to ride too.

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